In a compression ignition engine, combustion is ignited without the use of a separate spark, such as a spark plug. Instead, the combustion charge is ignited by compressing air alone, or a fuel and air mixture, in a cylinder. Air alone may be compressed until its temperature exceeds a critical level, after which fuel is injected and it ignites; or a fuel and air mixture may be compressed until the temperature reaches a critical point, at which point the fuel and air mixture ignites.
A well-known example of a compression ignition system is the diesel engine. Although diesel engines have been known and used for generations, there continues to be a need to improve the efficiency of the combustion and to reduce undesirable combustion by-products.
One problem associated with the known compression ignition engines is that there is a desire to use a high enough compression ratio to optimize efficiency; however, as the compression ratio increases there is a tendency for the fuel-air mixture to ignite prematurely. For instance, the combustion may commence on the upstroke, known as the compression stroke, rather than on the down stroke of the piston, which is known as the power stroke. Diesel systems have overcome this problem by introducing the fuel into the combustion chamber later in the compression stroke. Although this change may limit premature combustion, delaying the fuel injection leads to other difficulties in the combustion. For instance, when the fuel injection is delayed, the fuel may not properly mix with the air, which can result in poor combustion of the fuel and air mixture. Accordingly, there exists a need for a combustion system that can improve the efficiency of the combustion process.